Essentially never. I don't run dungeon crawls and traps in general bore me. My Eberron campaign is just had its 75th session and I doubt we've had even 7 sessions where I featured traps, so I voted 0%.
I mean, a percentage of what? How much time is spent on them? Percentage of encounters? Does it count if the PCs completely avoid an area with traps? Etc. . ?
My answer would be I use them as often as it makes sense to the context of an adventure/area.
Very seldom, because I haven't run many dungeon crawls -- probably two in the last three or four years. When I do run a dungeon adventure, like I am right now, I use traps quite a bit.
I think I'd use them more if my group featured a rogue.
I put 10%. If there is occasion for the players to break into a guarded facility, I might use a trap or two, but I don't have many random underground hideaways full of treasure and traps.
I don't really like the 3.5 trap rules. Most traps it's either going to kill the PC or it's not, so they're only meaningful if combined with some other danger. And if you calculate prices, the people who make these traps aren't getting much bang for their buck.
I also count as traps situations where taking the treasure activates the golem of doom or waving your fanny at the chained-up spider results in the chain snapping.
I said 10%. I only use traps were they are needed and that's rare. My last campaign the most common trap was a trip wire attached to a bell or other warning device.